Previous studies in experimental animals, and a more recent study in man, have indicated that fasting plasma insulin concentrations, as an estimate of insulin action in vivo, are well correlated with muscle phospholipid polyunsaturated fatty acid content. This offers a potential mechanism for the development of insulin resistance with the development of obesity. It is suggested that alterations in the fatty acid content of skeletal muscle phospholipids may change with increasing body fat content and there also affect insulin action. In this study we will examine the relationship of obesity, insulin action in vivo and skeletal muscle membrane phospholipid content in Pima Indians to determine if obesity causes insulin resistance by altering membrane phospholipid content.